1. Field of the Invention
The present invention relates to an optical instrument and an image photographing method. More particularly, the present invention relates to an optical instrument and an image photographing method for photographing an object image with appropriate color balance in indoor photography in particular.
2. Description Related to the Prior Art
Optical instruments, such as a photographic camera and a digital still camera, are widely used for photographing an object image. The photographic camera is used with silver halide photo film. The digital still camera incorporates a CCD image sensor. Any of such optical instruments is generally provided with an electronic flash device, which makes it possible to photograph an object indoors with insufficient illumination.
A photographic camera is used with a daylight type of color negative photo film which is the most suitable for outdoor photography under daylight. The flash device incorporated in the camera has color balance (color temperature) of flash light corresponding to color balance of daylight. Recently, there have been color negative photo films having high speed without lowering the image quality. As cameras of types with the flash device have been widely used, users takes indoor photographs the more frequently.
It is likely indoors that the use of flash light creates a high contrast frame in which an image of a principal object is over-exposed and recorded in a whitened manner, and/or an image of a background scene is under-exposed and darkened. To avoid such unwanted exposures, various suggestions have been made, including heightening speed of the photo film, lowering a shutter speed, and lowering the f-number of a photographic optical system. Also the principal object is illuminated with flash light being weakened, and the background scene is illuminated suitably by indoor illumination of an indoor light source.
However it is impossible to obtain a photographic print with sufficiently high color reproduction or color balance. When the principal object is effectively illuminated by flash light, the principal object is photographed on the photo film with coloration or tint near to that of its image which would be photographed while illuminated by daylight. The background scene behind the principal object is not illuminated by flash light, and thus photographed with the coloration of the indoor illumination. In general, the indoor illumination has color different from the daylight color, to cause a problem in the color reproduction. When the principal object is reproduced from the photo film on the photographic print neutrally or with the coloration or chromaticity very near to color of the photographic object, the color of the indoor illumination strongly remains in the background scene. In contrast, when the background scene is reproduced on the photographic print with neutral coloration, the color complementary to that of the indoor illumination strongly remains in the principal object. In any case, color failure occurs.
Let the indoor illumination be light of a tungsten lamp, the photo film of a daylight type be used under the indoor illumination, and the principal object be photographed while illuminated with flash light of the daylight color. When the photographic print is produced, the background scene has deep orange color while the principal object has its neutral coloration on the photographic print. If a printer of a type capable of correcting the colors effectively is used to produce the photographic print, the whole of the frame is finished with the coloration near to the neutral coloration. In such a frame, the background scene only has light orange color. But the cyan color is conspicuously added to the portion of the principal object, the cyan color being complementary to the orange color. An image of a person's face will be finished with very unnatural color reproduction.
If the indoor illumination is light of fluorescent lamps, pale green is added to an image of the background scene of the photographic print. Or the magenta color is conspicuously added to the image of the principal object for example a flesh-color portion of a person, the magenta color being complementary to the green color. A portrait of a person results in very unnatural color reproduction.
In order to prevent such problems, professional photographers selectively use photo films of a daylight type and a tungsten light source type. Also they use selectively lens filters suitable for the types of light sources for the purpose of obtaining optimized color balance. Those methods of indoor photography are well-known in the field of photography, but very difficult for amateur photographers to use. In addition, similar problems occurs with the digital still camera as optical instrument which uses the CCD image sensor instead of the silver halide photo film.